1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor used for forming an image on an electrophotographic principle, and an image forming apparatus provided with the electrophotographic photoreceptor.
2. Description of the Related Art
An image forming apparatus (hereinafter also referred to as “electrophotographic device”) on an electrophotographic principle that forms an image with use of an electrophotographic technology is diversely used as a copy machine, a printer, a facsimile machine, or the like.
The electrophotographic device is generally provided with an electrophotographic photoreceptor (hereinafter also referred to as “Photoreceptor”), an electrostatic charger, exposure means, image development means, transfer means, and fixing means.
The above-mentioned photoreceptor is generally constituted of: a conductive substrate comprising a conducting material; and a photosensitive layer comprising a photoconductive material laminated on the conductive substrate.
Examples of the above-mentioned photoreceptor include: an inorganic photoreceptor such as a selenium-based photoreceptor in which a photosensitive layer comprises amorphous selenium (a-Se) or amorphous arsenic selenium (a-AsSe), a zinc oxide-based photoreceptor in which a photosensitive layer comprises zinc oxide (ZnO), a cadmium sulfide-based photoreceptor in which a photosensitive layer comprises cadmium sulfide (CdS), an amorphous silicon-based photoreceptor in which a photosensitive layer comprises amorphous silicon (a-Si), and the like; and an organic photoreceptor in which a photosensitive layer comprises an organic photoconductive material, that is, an organic photoconductor (abbreviation: OPC).
The organic photoreceptor has slight problems concerning sensitivity, durability, stability to environments, and the like, but has many advantages, in comparison with the inorganic photoreceptor, in point of toxicity, manufacturing cost, freedom of material designing, and the like.
Also, the organic photoreceptor has a characteristic that a photosensitive layer can be formed by an easy and inexpensive method as represented by, for example, a dip coating method, and therefore it has been becoming mainstream of photoreceptors at present.
As a structure of such an organic photoreceptor, a variety of structures are proposed which include: a single-layer structure comprising both a charge generation material and a charge transport material (also referred to as “charge transfer material”) dispersed in a binding resin (also referred to as “binder resin” or “binding agent resin”), which is positioned on the conductive substrate comprising the conducting material; a laminated structure in which a charge generation layer comprising a charge generation material dispersed in a binding resin and a charge transport layer comprising a charge transport material dispersed in a binding resin are laminated on the conductive substrate in this order; a reverse-laminated structure in which the above-mentioned layers are laminated on the conductive substrate in a reverse order; and the like.
The organic photoreceptor having the laminated structure in which the charge transport layer is laminated on the charge generation layer is a functional separation type, and is excellent in electrophotographic property and durability. Also, the organic photoreceptor having the laminated structure can diversely utilize a photoreceptor property due to high freedom of material designing, and therefore it has been practically used extensively.
Although a laser printer is a typical example of the electrophotographic device in which a laser is an exposing source, a copy machine has been digitalized in recent years and thereby has commonly used a laser as an exposing source as well.
Among lasers used as an exposing source, a semiconductor laser has practically been used due to low cost, low energy consumption, lightweight, and compact size. Particularly, a semiconductor laser has commonly been used, having stability of an oscillation wavelength and an output, and a long lifetime due to the oscillation wavelength of around 800 nm in a near-infrared area.
A reason why such a semiconductor laser has commonly been used is that there was technical difficulty to practically use a laser which oscillates a laser beam with a wavelength shorter than the above-mentioned wavelength.
Therefore, as a charge generation material used in the electrophotographic device in which the semiconductor laser is the exposing source, an organic compound having sensitivity that light is absorbed in a long-wavelength area has generally been developed, and a laminated photoreceptor having a charge generation layer comprising the organic compound, particularly a phthalocyanine pigment, has been developed.
In the meanwhile, a manufacturing method of a blue light emitting diode (disclosed in Japanese Patent No. 2628404) was invented in 1990, and technologies related to a blue light semiconductor laser have vigorously been developed since then. Consequently, a next-generation disc, which is designated as a blue-ray disc that uses the technology of this blue light semiconductor laser, has quickly been widespread.
Also, heightening resolution of an image has been studied in recent years in order to improve quality of the image outputted from an electrophotographic device.
As a means of achieving the high resolution, i.e. a high record density, of the image, an optical method is exemplified, which is to narrow a spot diameter of a laser beam and to increase the record density.
On this account, a focal length of a lens used for narrowing the spot diameter of the laser beam needs to be shortened. However, design difficulty in terms of an optical system arises, and additionally it is difficulty to obtain clearness of a spot outline of the laser beam that has the oscillation wavelength of around 800 nm in the near-infrared area even if the spot diameter of the laser beam is narrowed by controlling the optical system. A reason why the clearness of the spot outline is difficult to be obtained is that diffraction of the laser beam is limited, and it is an inevitable phenomenon.
A spot diameter of a laser beam which is focused onto a peripheral surface of a photoreceptor can generally be calculated from an oscillation wavelength of the laser beam and a lens numerical aperture, and is represented by the following formula:D=1.22λ/NA wherein D represents the spot diameter, λ represents the oscillation wavelength of the laser beam, and NA represents the lens numerical aperture.
According to the above-mentioned formula, it is recognized that the spot diameter D is proportional to the oscillation wavelength of the laser beam. Therefore, a shorter spot diameter can be obtained by shortening an oscillation wavelength.
Accordingly, it is understood that the blue light semiconductor laser, instead of a near-infrared semiconductor laser which is mainly used at present, can actualize higher resolution of an image.
As a means of obtaining the high resolution of an image, a single-layer photoreceptor, instead of the laminated photoreceptor which is mainly used at present, can be used.
The laminated photoreceptor is generally constituted of the charge transport layer arranged on the surface side and the charge generation layer arranged on the substrate side. This arrangement is for the purpose of protecting the charge generation layer that is weak in film strength with the charge transport layer that has strong film strength due to a resinous component which is highly contained in the charge transport layer.
In the above-mentioned arrangement, a laser beam passes through the charge transport layer on the surface side and reaches to the charge generation layer, and then electrical charges are generated in the charge generation layer. Due to an electric field, one electrical charge flows toward the substrate side, and another electrical charge flows toward the surface side, and then an electrical charge on a peripheral surface of the laminated photoreceptor is eliminated. In such a case, some of the electrical charge in the charge transport layer are scattered about, and by the time these electrical charges reach to the photoreceptor surface, an electrical latent image is unclear. Unclearness of the electrical latent image increases with a thickness of the charge transport layer.
The single-layer photoreceptor, on the other hand, generates electrical charges in the vicinity of a peripheral surface thereof, and the electrical charges are not scattered about, and therefore an electrical latent image is clearly developed.
Laminated organic photoreceptors can be grouped into a negative electric type in which a charge transport material, which is a major functional component, is an electron-hole-transport material, and into a positive electric type in which a charge transport material is an electron-transport material.
With research and development of the organic photoreceptors, the electron-hole-transport material which has an excellent charge-transport function has swiftly been developed, and therefore the negative electric-type photoreceptor has practically been used. However, the negative electric-type photoreceptor has problems such that it generates harmful ozone and nitrogen oxides in large quantities, electrification cannot be uniformly made by a corona discharge, and the like.
On the other hand, the positive electric-type photoreceptor does not have the problems that the negative electric-type photoreceptor has. Also, the positive electric-type photoreceptor can adopt a process technology for a positive electric-type inorganic photoreceptor, such as a selenium-based photoreceptor and an amorphous silicon (a-Si) photoreceptor, and therefore a highly functional positive electric-type organic photoreceptor has strongly been desired. Japanese Patent No. 2718048 discloses a photoreceptor in which a diphenoquinone compound is used as a charge transport material having an electron-transport function, but the photoreceptor does not have adequate sensitivity since the diphenoquinone compound has a slow charge-transfer rate.
Hitherto developed positive electric-type organic photoreceptors include: a single-layer photoreceptor of U.S. Pat. No. 3,484,237 that is provided with a charge-transfer complex comprising polyvinyl carbazole (PVCz) and trinitrofluorenone (TNF); and a single-layer photoreceptor in which a charge generation material and an electron-hole-transport material are dispersed in a binding agent. The both photoreceptors are no longer used, since the former photoreceptor has low sensitivity, and TNF used therein is a carcinogenic material, and the latter photoreceptor has low sensitivity and charge retentivity, and an electric property which decreases after the photoreceptor is repeatedly used.
Japanese Unexamined Patent Application Publication No. HEI 9(1997)-240051 discloses a single-layer photoreceptor that adapts to a blue light semiconductor laser and comprises an α-type oxytitanium phthalocyanine pigment as a charge generation material.
Japanese Unexamined Patent Application Publication No. 2000-47408 discloses a single-layer photoreceptor that adapts to a blue light semiconductor laser and comprises a perylene-type compound as a charge generation material, but only discloses functions of a laminated photoreceptor. In the case where the charge generation material is the perylene-type compound, the photoreceptor cannot have adequate sensitivity in a short-wavelength area.
The single-layer photoreceptor disclosed in Japanese Unexamined Patent Application Publication No. HEI 9(1997)-240051 adapts to the blue light semiconductor laser as an exposing source, requires the charge generation material as an essential component, which is also required in conventional image forming apparatuses, and comprises the α-type oxytitanium phthalocyanine pigment, a dye, and the like as organic pigments.
Generally, a single-layer photoreceptor comprises in its entire peripheral surface layer both a charge generation material, which is a low-molecular compound, and a charge transport material. The single-layer photoreceptor therefore has a problem that wear resistance of the peripheral surface layer is weak, in comparison with wear resistance of a laminated photoreceptor that comprises only a charge transport material on the surface side.
Japanese Unexamined Patent Application Publication No. 2000-47408 discloses the single-layer photoreceptor comprising a styryl-type compound as an electron-hole-transport material, an electron transport material, and a phthalocyanine pigment as the charge generation material.
The single-layer photoreceptor of Japanese Unexamined Patent Application Publication No. 2000-47408 has problems that sensitivity is low in a short-wavelength area, wear resistance is weak, and the pigment itself, which generates electrical charges, functions as a trap during charge transport and thereby induces the low sensitivity and an increase of a residual potential after the photoreceptor is repeatedly used.
The present invention has an object of providing a single-layer photoreceptor and an electrophotographic device provided therewith, the single-layer photoreceptor having a high sensitivity behavior even in a long-wavelength area of a blue light semiconductor laser with exposure wavelengths of from 400 nm to 450 nm inclusive, being excellent in electric property and mechanical durability, and having high durability so as not to generate an abnormal image.